Parasitic oscillation problem untraceable

[TriodeLuvr]

Unbalancing the PI creates even-order harmonics.

How does a slightly lower amplitude on one side introduce harmonics?

The imbalance creates an asymmetrical waveshape. Asymmetry is composed of even-order harmonics.

[JMPGuitars]

Unbalancing the PI creates even-order harmonics. That's OK, but it's not related to his problem and it might make troubleshooting more difficult. If he builds the phase inverter exactly according to the original Marshall design, it can be eliminated as the culprit.

As an aside, it appears to me that the original 82K/100K design is already unbalanced. The correct values for a balanced PI appear to be 91K/100K. Maybe 100K/100K simply unbalances the PI in the opposite direction.

That is correct. I don't think it matters either way for his issue.

91K is correct, but 82K supposedly became the standard because it was a value more readily available. Honestly it's kind of silly. But also consider that they were using mediocre carbon comps whose values would stray up. So they could have found 82Ks that ran high fairly easily. If it's 10% tolerance, and straying up, then they could be very close to balanced if the 100Ks are accurate enough.

[TriodeLuvr]

LOL, you're absolutely right. It's easy to forget sometimes about the real tolerances with this type of circuitry. I'm old enough to remember when most schematics had a note at the bottom, something like "All voltages are +/- 20%." Modern circuitry with regulated supplies and semiconductor junctions is really OCD in comparison.

Jack

[Bieworm]

Well I think there is some progress. There is still fizzy on the OD but it's creamier. Perhaps it's just the nature of the amp?
The changes I made:

PI:
RK 470R + 22k tail

6V6's:
RK 330R + 1000uf cap (91.7%)
1k5 5W screen resistors

All in all sounds pretty good, but I'll have to play some more. It's pretty late now...

Here's the voltage chart.. is the negative grid voltage normal on V1? Nothing's plugged in the input.

Fwiw I checked the voltage swing on the cathode of the power tubes. Idee is 21.1V and with a good strum it rises up to 30V

16097961712413564128678924108376.jpg

Sound clip:
https://www.dropbox.com/s/er24jamf71c5q ... 4.m4a?dl=0

[TriodeLuvr]

Here's the voltage chart.. is the negative grid voltage normal on V1? Nothing's plugged in the input.

Both V1 grids should be grounded by the input jacks when nothing is plugged in. If you didn't use switched jacks, a small negative voltage might be normal. 1M to ground is probably high enough to allow the grid(s) to develop voltage due to contact potential. IOW, the 1M might be acting as a grid leak resistor, not just a grid resistor (the two terms are frequently misused).

Fwiw I checked the voltage swing on the cathode of the power tubes. Idee is 21.1V and with a good strum it rises up to 30V

At 30V, the amp is deep into Class B. That's almost certain to be the problem. You're hearing crossover distortion.

Jack

[Bieworm]

Here's the voltage chart.. is the negative grid voltage normal on V1? Nothing's plugged in the input.

Both V1 grids should be grounded by the input jacks when nothing is plugged in. If you didn't use switched jacks, a small negative voltage might be normal. 1M to ground is probably high enough to allow the grid(s) to develop voltage due to contact potential. IOW, the 1M might be acting as a grid leak resistor, not just a grid resistor (the two terms are frequently misused).

Fwiw I checked the voltage swing on the cathode of the power tubes. Idee is 21.1V and with a good strum it rises up to 30V

At 30V, the amp is deep into Class B. That's almost certain to be the problem. You're hearing crossover distortion.

Jack

It's a single input cliff jack. Grounded when nothing's plugged in...
PT bias is 92% now... so raising that more???

[TriodeLuvr]

[TriodeLuvr]

It's a single input cliff jack. Grounded when nothing's plugged in...

If it's grounded, and the grid is connected to it, the grid can't have standing DC. Check it with an ohmmeter.

PT bias is 92% now... so raising that more???

No, that won't fix it. The issue is that bias is changing when you apply signal. The only solution is fixed bias, either at the cathode or at the grid. You can test it the way I suggested earlier. Just make sure the batteries are securely connected. You wouldn't want to lose bias once the cathodes are grounded. Set the 100K pot so you have -20V on the grids, and you'll be OK. Once you've confirmed this is the problem, you can decide how to proceed.

Jack

[JMPGuitars]

It's a single input cliff jack. Grounded when nothing's plugged in...

If it's grounded, and the grid is connected to it, the grid can't have standing DC. Check it with an ohmmeter.

PT bias is 92% now... so raising that more???

No, that won't fix it. The issue is that bias is changing when you apply signal. The only solution is fixed bias, either at the cathode or at the grid. You can test it the way I suggested earlier. Just make sure the batteries are securely connected. You wouldn't want to lose bias once the cathodes are grounded. Set the 100K pot so you have -20V on the grids, and you'll be OK. Once you've confirmed this is the problem, you can decide how to proceed.

Jack

That's not the only solution. It's just the one you prefer.

[dbharris]

For whatever reason, the dropbox link wouldn't work for me so I couldn't hear the problem. It seems to be awfully similar to the issue that I had with my 6V6 Plexi recently. It ended up being what Jack is describing, and I fixed it with the Paul Ruby Mod. See page 1 of this thread viewtopic.php?f=4&t=25732 for the cross over distortion on my scope prior to and after the mod (and after trying lots of other things that didn't help, but certainly didn't hurt either).

-Dan

[JMPGuitars]

For whatever reason, the dropbox link wouldn't work for me so I couldn't hear the problem. It seems to be awfully similar to the issue that I had with my 6V6 Plexi recently. It ended up being what Jack is describing, and I fixed it with the Paul Ruby Mod. See page 1 of this thread viewtopic.php?f=4&t=25732 for the cross over distortion on my scope prior to and after the mod (and after trying lots of other things that didn't help, but certainly didn't hurt either).

-Dan

The PR mod is a good option to try, and Bieworm knows how to do it (though he'll need a higher voltage zener). The Snubber circuit could also help. A couple other possibilities too. There's more than one way to skin a walrus.

[TriodeLuvr]

The PR mod is a good option to try,

That's exactly what I said - it's necessary to stabilize the bias either at the cathode or grid. Installing Zeners would stabilize the bias at the cathode.

Did Marshall use self-bias with 6V6s in PP? You can't just throw any tube type into the mix without re-engineering the design. EL84s run close to Class A in many older amps, and those designs are less prone to this problem.

I'm curious, being as the schematic that Bieworm posted in this thread shows the 6V6 outputs with fixed grid bias. How did the change to a self-biased version come about?

Jack

[Bieworm]

The PR mod is a good option to try,

That's exactly what I said - it's necessary to stabilize the bias either at the cathode or grid. Installing Zeners would stabilize the bias at the cathode.

Did Marshall use self-bias with 6V6s in PP? You can't just throw any tube type into the mix without re-engineering the design. EL84s run close to Class A in many older amps, and those designs are less prone to this problem.

I'm curious, being as the schematic that Bieworm posted in this thread shows the 6V6 outputs with fixed grid bias. How did the change to a self-biased version come about?

Jack

Because I had this fizz problem to begin with. I thought the PI gave too much gain causing this fizz.

[Bieworm]

For whatever reason, the dropbox link wouldn't work for me so I couldn't hear the problem. It seems to be awfully similar to the issue that I had with my 6V6 Plexi recently. It ended up being what Jack is describing, and I fixed it with the Paul Ruby Mod. See page 1 of this thread viewtopic.php?f=4&t=25732 for the cross over distortion on my scope prior to and after the mod (and after trying lots of other things that didn't help, but certainly didn't hurt either).

-Dan

The PR mod is a good option to try, and Bieworm knows how to do it (though he'll need a higher voltage zener). The Snubber circuit could also help. A couple other possibilities too. There's more than one way to skin a walrus.

That's what I was thinking too. So I re-added the PR mod that I already tried earlier. I figured the 24V zeners I had should do the job. But to no avail...

[crgfrench]

The imbalance creates an asymmetrical waveshape. Asymmetry is composed of even-order harmonics.

OK, I am going to postulate something everyone will probably hate here. Asymmetry does not necessarily imply any harmonics at all. I believe a lot of folks have simply taken Merlin's statement (from his discussion of the AC coupled LTP)...
"the inverting output will be 6% higher than the non-inverting output. This could be corrected by making Ra1 6% smaller in value, but in practice it is not necessary; a slightly unbalanced phase inverter is often quite benificial [sic] to guitar tone, due to the additional 2nd harmonic it introduces."
...as gospel without actually thinking about whether it's accurate. All the higher amplitude of one side does is make the height of the tops larger than the depths of the troughs (or vice versa). It merely attenuates one side of the fundamental first order harmonic. That is very different than adding a 2nd, 3rd, 4th or higher order harmonic to the fundamental! Merlin was just plain mistaken when he claimed that a second harmonic is introduced. There is no harmonic introduced. The fundamental is merely slightly quieter on half of the waveform; it's not shifted and no harmonic is added. Take a look at the image attached. Do ANY of these 2nd, 3rd or 4th harmonic output waveforms occur with a mismatched PI? NO. That's because the PI doesn't add harmonics through mismatched plate voltages; rather, it just makes the 1st order fundamental slightly asymmetric.

Screen Shot 2021-01-04 at 11.32.14 PM.png

[TriodeLuvr]

All the higher amplitude of one side does is make the height of the tops larger than the depths of the troughs (or vice versa). It merely attenuates one side of the fundamental first order harmonic.

I haven't read Merlin (Valve Wizard?) on this topic. I'm too newbie for that.

Anyway, when the "top half" of the sine is larger or smaller than the "bottom half," the composite waveform is no longer a pure sine wave. This can be verified empirically with a spectrum analyzer and an unbalanced phase inverter or (more accurately) an arbitrary waveform generator. It can also be confirmed with SPICE simulators and mathematically with Mathcad.

I no longer have access to Mathcad, but I can perform SPICE simulations of balanced VS unbalanced phase inverters and display the resulting FFT plots. I'm pretty sure this will confirm my statement that asymmetry creates even-order products.

Jack

[TriodeLuvr]

I figured the 24V zeners I had should do the job. But to no avail...

24V is probably too much bias (too cold).

Sorry if I've posted this link before. It describes another way to fix-bias the voltage at the cathodes, and it's adjustable. No relation to me or any of my projects.

http://tronola.com/A_New_Look_At_An_Old_Friend.pdf

[crgfrench]

...when the "top half" of the sine is larger or smaller than the "bottom half," the composite waveform is no longer a pure sine wave.

If it's coming from a stratocaster it's not a pure sine wave anyway. But let's work with a signal generator. You seem to believe that an "impure" sine wave implies there are higher order harmonics at play. That is not necessarily true.

[TriodeLuvr]

...when the "top half" of the sine is larger or smaller than the "bottom half," the composite waveform is no longer a pure sine wave.

If it's coming from a stratocaster it's not a pure sine wave anyway. But let's work with a signal generator. You seem to believe that an "impure" sine wave implies there are higher order harmonics at play. That is not necessarily true.

It is true in the context of our discussion involving the LTP.

Jack

[Bieworm]

I figured the 24V zeners I had should do the job. But to no avail...

24V is probably too much bias (too cold).

Sorry if I've posted this link before. It describes another way to fix-bias the voltage at the cathodes, and it's adjustable. No relation to me or any of my projects.

http://tronola.com/A_New_Look_At_An_Old_Friend.pdf

I thought the PR mod zener value was determined by taking the cathode voltage and add a few volts. So the 21V cathode voltage + a few volts would mean that my 24V zeners should be ideal? Or am I missing something in this situation?
It would be a lot easier to determine the cause of the fizz when I have my siglent 1102 to rule out the possibility that the fizz winds up to be a parasitic oscillation earlier in the circuit and gets amplified anyway. gosh!!! Or is the voltage swing on the cathode of them 6V6's a dead giveaway it IS crossover distortion?